BACKGROUND:
although clinical use of the click stimulus for the evaluation of brainstem
auditory function is widespread, and despite the fact that several researchers
use such stimulus in studies involving human hearing, little is known about
the auditory processing of complex stimuli such as speech.AIM: to characterize the findings of the Auditory Brainstem Response
(ABR) performed with speech stimuli in adults with typical development.METHOD: fifty subjects, 22 males and 28 females, with typical development,
were assessed for ABR using both click and speech stimuli.RESULTS: the latencies and amplitudes of the response components onset
(V, A and complex VA), the area and slope that occur before 10 ms were identified
and analyzed. These measurements were identified in all of the studied subjects
and presented wave latency values (ms) of: V = 7.18 (SD = 1.08), A = 8.66 (SD
= 1.13); Complex VA = 1.49 (SD= 0.43). For the wave amplitudes (µV), the
values were: V = 0.29 (SD = 0.15), A = -0.3 (SD = 0.18); Complex VA = 0.58 (SD
= 0. 25). The area measurements (µV X ms) and slope (µV / ms) were
0.27 (SD = 0.17) and 0.4 (SD = 0.17) respectively.CONCLUSION: based on the gathered data it can be observed that this
potential works as a new tool for understanding the encoding of sound at the
brainstem level.

The Auditory Brainstem Response (ABR) is generated by the
synchronism of auditory pathway structures. The ABR initiates in
the auditory nerve and continues through the cochlear nucleus,
superior olivary complex, lateral lemniscus up to the inferior
colliculus. In clinical practice, the most frequently used
acoustic stimulus is the click because it triggers synchronous
response of several neurons and exhibits a broad spectrum of
frequencies1. However, other types of stimuli, such as pure tones
and speech stimuli 2-3 can also be used to trigger electrical
responses.

The speech signal - complex spectral-temporal structure -
requires a synchronized neural response for accurate encoding.
The evoked responses exactly depend on this type of synchronous
activation and are ideal for studying the neural basis of speech
perception4.

The speech perception involves several processes - such as
peripheral auditory analysis and extraction of automated
characteristics in the brainstem nuclei (SCT) - that lead to the
classification of words and phonemes 4.

Thus, the brainstem responses generate direct information on
how the speech syllable sound structure is encoded in the
auditory system. The ABR with speech stimuli can be divided into:
transient and sustained portions, onset response components
(stimulus onset), and frequency-following response (FFR). The
onset responses are transient processes, similar to the click,
with tenths of milliseconds precision. They primarily represent
the response to discrete stimulus events, as during the
initiation, and the successive modulations caused by the
vibration of the vocal folds. The components of sustained
response continue during the reproduction of a periodic stimulus
and reflect the overall integrity of the response in relation to
the stimulus3.

Recent studies have reported alterations in brainstem
responses to speech stimulus in children with learning
disabilities and auditory processing disorders3,5-8, as well as
the relation to cortical processing9-11 and the efficiency of
auditory training in the rehabilitation of individuals with
speech perception deficits 5, 12.

Abrams and colleagues7 suggest that delays in latencies of
brainstem responses for speech stimuli have a negative impact on
the rapid processing of acoustic signals by specialized cortical
structures. Similarly, Wible and colleagues (2005) concluded that
the impairment of speech processing at the brainstem and cortical
level may be an indicator of physiological mechanisms
alterations, which may be responsible for an abnormal perception
of speech and thus compromise language abilities.

Song and colleagues13 explored the relationship between the
click stimulus and the speech stimulus in ABR in children with
and without learning disabilities. They concluded that the
responses obtained by the two stimuli reflect separate neural
processes and only the processes involved in encoding complex
signals (speech) are altered in children with learning
disabilities.

Therefore, it is necessary to study the auditory
representation and perception of speech in individuals with
typical development establishing reliable procedures and
normative values in order to determine the encoding of speech
stimuli in brainstem. This would enable the comparison of such
normative values to findings in individuals with Auditory
Processing Disorder (APD), learning disabilities, and language
disorders. This comparison would provide information of great
importance both for a differential diagnosis and for
rehabilitation.

This study aimed to characterize the findings of ABR performed
with speech stimuli in adults with typical development.

The speech stimuli used in this study (syllable /da/) was
produced at the Radio Laboratory of the School of Communication
and Arts, University of São Paulo (USP); the equipment
used in production were: Newman 189 microphone; MACKIE/SR32-4
mixer; M-AUDIO/1010LT sound card; Sound Forge 6.0 software(Sony);
Vegas 4.0 editing software (Sony).

The syllable /da/ was narrated by a male voice and edited to
produce the stimulus according to the parameters described by
King9 and Wible14. Only the first five formants were separated
from the original syllable, resulting in a 40ms stimulus that
contains the transient portion of it; the vowel /a/ was shortened
to allow for the increased rate of stimulation and thus better
activate the system.

The stimuli were organized into groups of four, separated by
12 ms, and the interval between each group of stimuli was of 30
ms.

Procedures

The research project and the consent form were reviewed and
approved by the Ethics Committee on Research of the University
Hospital of University of São Paulo, protocol 527/04.

Audiometric assessment, behavioral assessment of auditory
processing, and electrophysiological (ABR) assessment were
carried out in a quiet environment.

3.3. Electrophysiological assessment: ABR with click and
speech stimuli with the two channel equipment GSI-Audera. After
cleaning the skin with abrasive paste, the electrodes were placed
at the vertex, and right and left mastoid positions; electrolytic
paste and adhesive tape were used. The impedance values of
electrodes were below 5 K s.

The click stimulus was used to confirm the integrity of the
auditory pathway. The stimulus was presented to the right ear
with insertion earphone at a rate of 19 stimuli per second. A
total of 2000 stimuli at 80 decibels hearing level were presented
with a write window 10 milliseconds. A second stimulation was
performed in order to reproduce and confirm the trace of the
waves.

For the assessment with speech stimulus, CD player and
headphones (CV320-Coby) were used; the stimulus was presented to
the right ear at 75 decibels hearing level, with a rate of 11
stimuli per second. A total of 2000 stimuli were presented - two
scans of 1000 stimuli - with recording window of 50
milliseconds.

The tracings obtained from each scan were summed and the
components of the response onset that occur before 10
milliseconds (V, A, and VA complex) were identified in the
resulting trace.

Results

The ABR waves to clicks were analyzed for values of absolute
latencies and interpeaks of waves I, III and V in order to verify
the integrity of the auditory pathway according to normal
parameters proposed by Hall17.

For ABR with speech stimuli, the components of the onset
response (V, A, and VA complex) that occur before 10 ms were
identified and latency and amplitude values were analyzed. The VA
complex was investigated by measurements of latency, amplitude,
area and slope (VA amplitude / VA duration).

The values of both stimuli were statistically analyzed. Mean,
standard deviation, minimum and maximum values, and Pearson
correlation were calculated.

ABR with speech stimuli

Table
1 shows the latency and amplitude measures obtained at the onset response
to the waves V, A and VA complex, with the mean, standard deviation, minimum,
and maximum values.

Table
2 displays the values of mean, standard deviation, maximum, minimum and
Pearson correlation of the area and slope of the VA complex.

Analyzing the correlation, a possible positive moderate
correlation between measures of area and slope is verified in the
studied group.

Comparison between different stimuli

In Figure
1, scatter plot A represents the correlation between the latency of wave
V for the click stimulus and for the speech stimulus, and scatter plot B displays
the comparison of wave V obtained in ABR with click stimuli and ABR with speech
stimulus to each individual.

No correlation was observed between wave V of ABR with click
stimulus and wave V with speech stimuli (r = 0.0035).

Discussion

This study aimed to describe and characterize the findings of
ABR with speech stimuli in adults with typical development and
thus provide a possible normalization of such responses. This
data can be used in the assessment of the integrity of speech
signal encoding in normal individuals and individuals with
alterations.

Measures of latency
and amplitude were identified in the onset, slope, area and amplitude of VA
complex response components (Tables 1 and
2). Such measures are used to describe the
brainstem neural activity for speech, characterized by rapid temporal changes
and complex spectral distributions 3.

Latency measures generate information about the precision with
which the brainstem nuclei synchronously respond to acoustic
stimuli. In contrast, amplitude measures generate information
about how robust the response of the brainstem nuclei for the
acoustic stimulus is. Alterations in these measures might
indicate a difference in conduction speed along the dendrites and
axon projections, or a difference in kinetic channels of neurons,
or even differences in the synchronization of the response
generators.

These measures were identified in all individuals evaluated in
the current study. However, unlike the results found in other
studies which demonstrated highly reproducible results with the
waves and stability at test-retest situations 3,9,12, the
measures of the current study presented committed wave
reproducibility.

This instability probably occurred in the current study due to
the use of an edited natural speech signal, rather than a
synthesized speech signal, which is commonly found in ABR
research in other countries. The use of synthesized speech,
instead of natural speech, allows the precise manipulation and
modification of these dimensions that are difficult to control in
natural speech4.

The VA complex
measures inform about the timing of neuronal discharges18. Values of slope and
area of the VA complex were calculated by measuring the latency and amplitude
of VA complex (Table 2) according Russo3.
The authors interpret area measurements as the amount of activity that contributes
to the wave generation and slope as the temporal synchronization of the response
generators.

In the same study, results showed lower responses of slope in
children with learning disabilities when compared to normal
children. This finding corroborates with other studies6, 13.

There was no correlation
between wave V to click and wave V to speech stimuli (Figure 1
- scatter plots A and B) probably because the responses obtained by the two
different stimuli reflect neural processes. The speech signal contains different
acoustic information than the click stimulus, which adds information about the
neural coding at the brainstem level13. The processing that occurs in this region,
for both types of stimuli (click and speech), will reflect different pathways
on cortical processing11. Song and colleagues13 showed that a delay in measures
of ABR with speech stimuli does not necessarily cause a delay in measures with
click, and that only the processes involved in encoding complex signals are
altered in children with learning disabilities.

Thus, the responses obtained in the brainstem for both stimuli
provide additional and objective information on the encoding of
sound in the auditory system.

The data provided here serve as a measure to determine the
normal function of brainstem in response to speech stimuli.

Deficits in the latency and amplitude of the brainstem
response to speech stimuli have been found in some children with
learning disabilities 3, 5-7, 9, 13.

Another possible application of this potential refers to its
use in monitoring the auditory training; it demonstrates the
existence of brainstem plasticity and the efficiency of such
training, and in the rehabilitation of individuals with speech
perception deficits5, 12.

Measures of latency and amplitude of the brainstem responses
to speech stimuli can provide information about the neural
encoding for speech sounds. The analysis of the responses
obtained for individuals with typical development allowed an
objective estimation of normative values for the studied
potential.

Conclusion

Analysis of ABR responses obtained with speech stimuli in the
sample of individuals with typical development showed latency
(ms) results for waves V, A and complex VA with the following
values: V = 7.18 (SD = 1.08), A = 8.66 (SD = 1.13); VA Complex =
1.49 (SD = 0.43). For the amplitudes (µV) of the waves, the
values were: V = 0.29 (SD = 0.15), A =- 0.3 (SD = 0.18); VA
Complex = 0.58 (SD = 0 25). The area measurements (µV x ms)
and slope (µV/ms) were 0.27 (SD = 0.17) and 0.4 (SD = 0.17)
respectively. It is confirmed from the literature and from data
presented that this potential is a new tool for understanding the
encoding of sounds at the brainstem level. Furthermore, this
potential can provide important information about the mechanisms
and neural bases of normal and altered auditory function as it
shows quantifiable measures of neural encoding of speech sound,
regardless of the attention of the individual.